The only way i know of doing that is to actualy create a little verticaly tileing image. So what youd do is start with like a 4x8 pixel image that has the same background color as whatever is outside the table, create a gradient across this little image using black, make sure you end the gradient completely before hitting the edge of the little image otherwise youll get a funny line instead of a shadow, then create a colum on each side of the table and tile your little shadow jpeg verticaly. kinda hard to explain.